Barleywine
Can I ask you how you see/what you feel the traditional meaning is for it? Because other books and online sites I've peeked at seem to agree with what Anna has.
Sure! From the Phillipe Lenormand sheet: "If near, means the certain success of a wish or a plan; if distant, the contrary."
Otherwise, a "Yes" answer; "It will come to pass;" certainty; providence.
It implies a dispensation received in an unambiguous way, not one that has to be unlocked. Too much emphasis can be placed on the presumed "action" associated with a key, and not the simple fact that "the answer is Yes." It feels more like a "get in free"door pass than the solution to a riddle.
It seems to me to be less about seeking an answer by having to go through some dedicated "unlocking" process (with the associated psychological assumptions) than about having the affirmative answer handed to you. I would look at the surrounding card to see what the context of "Yes" might be, but I don't read too much that's functionally literal into the "key" image or metaphor.